avatar-chooser: Return a proper widget if images cannot be loaded

GTK assumes that a widget will be created for a flow box item. So always
return one instead of falling back to returning NULL. This can primarily
happen if an image loader is missing.
7 jobs for benzea/fix-criticals-missing-image-loader in 1 minute and 55 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #926204
build

00:01:14

 
  Test
passed #926205
test

00:00:40

 
  Manual
manual #926207
privileged allowed to fail manual
asan
manual #926206
allowed to fail manual
flatpak
manual #926210
allowed to fail manual
msan
manual #926208
allowed to fail manual
tsan
manual #926209
allowed to fail manual
ubsan